Allow app_shell embedders to override extensions client interfaces

extensions/shell/renderer/shell_content_renderer_client.cc

Index: extensions/shell/renderer/shell_content_renderer_client.cc
diff --git a/extensions/shell/renderer/shell_content_renderer_client.cc b/extensions/shell/renderer/shell_content_renderer_client.cc
index 216eac60424c91ce5e6af2239ae1379e78e83deb..a56c009d0be650a608d6fe8300f0e0c93f02982f 100644
--- a/extensions/shell/renderer/shell_content_renderer_client.cc
+++ b/extensions/shell/renderer/shell_content_renderer_client.cc
@@ -79,7 +79,7 @@ ShellContentRendererClient::~ShellContentRendererClient() {
 void ShellContentRendererClient::RenderThreadStarted() {
   RenderThread* thread = RenderThread::Get();
 
-  extensions_client_.reset(new ShellExtensionsClient);
+  extensions_client_.reset(CreateExtensionsClient());
   ExtensionsClient::Set(extensions_client_.get());
 
   extensions_renderer_client_.reset(new ShellExtensionsRendererClient);
@@ -189,4 +189,8 @@ ShellContentRendererClient::CreateBrowserPluginDelegate(
   }
 }
 
+ExtensionsClient* ShellContentRendererClient::CreateExtensionsClient() {
+  return new ShellExtensionsClient;
+}
+
 }  // namespace extensions